Problem 5.98

A point source of light with wavelength $\lambda=0.50 \mu \mathrm{m}$ is located at a distance $a=100 \mathrm{~cm}$ in front of a diaphragm with round aperture of radius $r=1.0 \mathrm{~mm} .$ Find the distance $b$ between the diaphragm and the observation point for which the number of Fresnel zones in the aperture equals $k=3$.